The reliability of computer based applications continues to be an important consideration. Moreover, in fault-tolerant computing applications, insuring that a fault-tolerant pair of CPUs agree on a result, and transmit the agreed-to result to a network (e.g., an Ethernet bus) is a critical function. Several prior techniques have been used to perform this function, but all have certain deficiencies.
As an example, turning to FIG. 1, two CPUs, namely CPU A 11 and CPU B 13 redundantly process information. Communications subsystems A 15 and B 17 are attached to CPUs A 11 and B 13, respectively. In one example, communications subsystems 15 and 17 may implement NodeBus/RS-232 communications. Each communications subsystem is attached to a synchronous bit comparator 19 using data and synchronization (e.g., RTS/CTS) connections. Comparator 19 performs a bit-by-bit comparison of the data streams, keeping them in sync using the synchronization connections. If the comparison agrees, the data is transmitted, bit-by-bit, to communications network 21. Network data is commonly received by the two communications subsystems and verified by CPU operations.
The technique depicted in FIG. 1, and described above, requires precise bit synchronization between the two communications subsystems. This synchronization is implemented using the synchronization signaling between the communications subsystems and synchronous bit comparator 19. Unfortunately, this technique is not generally applicable to many communications protocols because they do not provide facilities for the synchronization signaling required by this technique.
Turning to FIG. 2, a system employing another data comparison technique is depicted. In this example system, a message from CPU A 11 is formatted by its respective communications subsystem 15 and transmitted to communications subsystem 17 of CPU B 13. Communications subsystem B 17 compares the message received from communications subsystem A to the message that it intends to transmit. If the comparison agrees, the message is transmitted to network 21. One drawback to this technique is the requirement that each message be transmitted twice, with associated delays. That is, the message must first be transmitted from the first communications subsystem to the second communications subsystem, compared, and then transmitted from the second communications subsystem to the destination network.
The present invention is directed toward solutions to the above-described problems.